Mechanistic Insights into the Proton Transfer and Substitution Dynamics of N-Atom Center Reactions: A Study of CH3O- with NH2Cl.

Abstract

Bimolecular substitution reactions involving N as the central atom have continuously improved our understanding of substitution dynamics. This work used chemical dynamics simulations to investigate the dynamics of NH2Cl with N as the central atom and the multiatomic nucleophile CH3O- and compared these results with the F- + NH2Cl reaction. The most noteworthy difference is in the competition between proton transfer (PT) and the SN2 pathways. Our results demonstrate that, for the CH3O- + NH2Cl system, the PT pathway is considerably more favorable than the SN2 pathway. In contrast, no PT pathway was observed for the F- + NH2Cl system at room temperature. This can be attributed to the exothermic reaction of the PT pathway for the CH3O- + NH2Cl reaction and is coupled with a more stable transition state compared to the substitution pathway. Furthermore, the bulky nature of the CH3O- group impedes its participation in SN2 reactions, which enhances both the thermodynamic and the dynamic advantages of the PT reaction. Interestingly, the atomic mechanism reveals that the PT pathway is primarily governed by indirect mechanisms, similar to the SN2 pathway, with trajectories commonly trapped in the entrance channel being a prominent feature. These trajectories are often accompanied by prolonged and frequent proton exchange or proton abstraction processes. This current work provides insights into the dynamics of N-centered PT reactions, which are useful in gaining a comprehensive understanding of the dynamics behavior of similar reactions.